Satellite communication systems are widely used in both military and civilian applications. Most civilian communication satellites are in geostationary orbit 22,300 miles above the earth. They receive signals from a first earth station, such as a portable terminal, and then amplify and translate the signal in frequency prior to retransmitting the signal to a second earth station. This process makes effective use of line of sight microwave bands without requiring relay towers.
The most common microwave bands used by civilian communication satellites are the carrier bands of 5,925 MHz to 6,425 MHz in the uplinks and 3,700 MHz to 4,200 MHz in the downlinks. Using microwave bands in an uplink, however, requires more complicated circuitry because of the high frequencies involved. In addition, because geostationary satellites are stationed in a high orbit with respect to other satellites, a high power output from a transmitting terminal is required. Thus, a portable terminal's size, circuitry, and cost may be elevated when operating in microwave bands, rather than when operating in lower frequencies.
An alternative satellite communication system, known as ORBCOMM, has been proposed by the Orbital Communications Corporation of Fairfax, Virginia. The ORBCOMM system is described in a brochure published by the Orbital Communications Corporation entitled "ORBCOMM.SM., Vital Communications Absolutely Anyplace on Earth.SM." (1992). The ORBCOMM system is designed to bring data communications and position determination to a multitude of mobile portable terminals. The ORBCOMM network provides low speed VHF digital data communications using low earth orbiting satellites that have a very high availability. As shown in FIG. 1, the basic ORBCOMM system uses a network of satellites 111, a network control station 112, and a base station 113 to communicate with portable terminals 110. The portable terminals 110 can be pocket sized with a user input and display, and can operate for long periods on battery power. A typical message of around 100 bytes or characters is transmitted from a portable terminal 110 through the satellite 111 and to the network control center 112. The data can then be stored at the network control center 112 and accessed at a customer's convenience, similar to electronic mail. A customer at a base station 113, which could be a portable computer with a MODEM, can then poll the network control center 112 for messages via a standard phone line 117.
One element of the portable terminal 110 is an amplifier stage. The output of an amplifier in an amplifier stage is usually classified into a commonly recognized class depending on the conduction characteristics of that amplifier's active devices. One of the most common classes of amplifier is the class C amplifier. Conventional low voltage high power (i.e. class C) amplifiers, however, typically have poor power added efficiency. This is true, in part, because of conduction losses from the relatively large currents required to develop sufficient RF output power from a low voltage supply (e.g. 7 V). Another problem associated with class C amplifiers is that they tend to be phase non-linear. In other words, amplitude variation of the output signal induces a phase change in the output signal, for example, when a class C amplifier is hard limited. Because class C amplifiers tend to be hard limited, and thus phase non-linear, class C amplifiers tend to cause spectral regrowth, or intermodulation, problems. Spectral regrowth occurs when side frequency lobes of phase modulated signals experience phase nonlinearity conditions and then mix with themselves, which can potentially cause co-channel interference. Thus, spectral regrowth may hinder reception in phase modulated systems. Class C amplifiers are also more sensitive to device parameters. For example, transistors may not be easily substituted, even though their data sheet characteristics are similar. This sensitivity can make coupling class C amplifiers in stages more complicated. These characteristics of class C amplifiers make them poor choices for portable terminals when communicating with a low earth orbiting satellite.